Generally, in coating materials used for electrostatic coating of vehicle bodies, etc., there is a solvent-based coating material (oil-based coating material) whose electric resistance is relatively high, an aqueous coating material (water-based coating material) whose electric resistance is relatively low, and a metallic coating material in which metallic powder is dispersed in the above coating materials. Of these, where carrying out electrostatic coating using an aqueous coating material or a metallic coating material whose electric resistance is relatively low, a current is caused to flow to the ground via a coating material feeding channel and a coating material tank if high voltage is applied directly to a charge electrode of an electrostatic coating spray gun which is brought into contact with the coating material. Therefore, no electric discharge is brought about between the charge electrode and a substance to be coated, wherein atomized coating material particles cannot be electrified.
As a prior art to solve the problem, for example, there is a method for electrically insulating a coating material tank from the ground. According to the method, high voltage can be applied between a charge electrode of an electrostatic coating spray gun and a substance to be coated, wherein coating material particles can be electrified. However, it is necessary that painting or coating work is interrupted when supplementing a coating material since high voltage is applied to the coating material tank, or a special coating material supplementing apparatus (for example, refer to JP-A-2002-143730) is required, which supplies a coating material in a state where electric insulation from the coating material tank is maintained. Therefore, it is inconvenient.
As another solving means, there is a system called an “external electrode system” in which one or a plurality of external electrodes is (are) disposed at an outward position in the diametrical direction from an electrostatic coating spray gun, and high voltage is applied thereto. In this system, there is a system (for example, refer to JP-A-H06-134353) in which a rotary atomizer head is used to atomize a coating material in an electrostatic coating spray gun, and an air spray system (for example, JP-A-H09-136047) in which compressed air is used. In both systems, since there is no case where an external electrode for applying high voltage is brought into contact with a coating material whose electric resistance is low, it is possible to electrify coating material particles with the coating material tank grounded. Accordingly, no special apparatus is required to supply a coating material into a coating material tank, wherein continuous coating is enabled.
However, since, in the case of the external electrode system, the external electrode is attached outside an electrostatic coating spray gun, the electrostatic coating spray gun is made large-sized, and this is dangerous because the electrode to which high voltage is applied is provided outside the main body. In addition, there is another problem in that atomized coating material particles are adhered to the vicinity of the external electrode or the surrounding of the electrostatic coating spray gun due to an electrostatic force.